October 30, 2013

The atomic arrangement of an InGaN/GaN interface created by layer-by-layer atomic crystal growth. The new technique may lead to new developments in solar-cell efficiency. (Credit: Arizona State University)

Arizona State University and Georgia Institute of Technology researchers have developed a new approach to growing indium gallium nitride (InGaN) crystals, promising “record-breaking” photovoltaic solar cell efficiencies.

Researchers previously found that the atomic separation of the crystal layers of the InGaN alloy varies, which can lead to high levels of strain, breakdowns in growth, and fluctuations in the alloy’s chemical composition.

“Being able to ease the strain and increase the uniformity in the composition of InGaN is very desirable, but difficult to achieve,” said Arizona State University physics professor Fernando Ponce.

“Growth of these layers is similar to trying to smoothly fit together two honeycombs with different cell sizes, where size difference disrupts a periodic arrangement of the cells.”

Controlled crystal-layer growth

So the researchers developed a “metal-modulated epitaxy” approach that allows controlled atomic layer-by-layer growth of the material. Thin films grown with the epitaxy technique had almost ideal characteristics; the unexpected results came from strain relaxation at the first atomic layer of crystal growth.

The final crystal is more uniform and the lattice structures match up, resulting in a film that resembles a perfect crystal. “The luminosity was also like that of a perfect crystal. Something that no one in our field thought was possible.”

Eliminating these two defects (non-uniform composition and mismatched lattice alignment) ultimately means that solar photovoltaic devices (and LEDs) can now be developed that have much higher, more efficient performance, according to the researchers.

The InGaN alloy also forms the light emitting region of LEDs for illumination in the visible range and of laser diodes (LDs) in the blue-UV range.

Abstract of Applied Physics Letters paper

We have studied the properties of thick In x Ga1− x N films, with indium content ranging from x ∼ 0.22 to 0.67, grown by metal-modulated epitaxy. While the low indium-content films exhibit high density of stacking faults and dislocations, a significant improvement in the crystalline quality and optical properties has been observed starting at x ∼ 0.6. Surprisingly, the In x Ga1− x N film with x ∼ 0.67 exhibits high luminescence intensity, low defect density, and uniform full lattice-mismatch strain relaxation. The efficient strain relaxation is shown to be due to a critical thickness close to the monolayer range. These films were grown at low temperatures (∼400 °C) to facilitate indium incorporation and with precursor modulation to enhance surface morphology and metal adlayer diffusion. These findings should contribute to the development of growth techniques for nitride semiconductors under high lattice misfit conditions.

Comments (10)

Is this similar to the spaceship material supposedly found at Roswell? Can it be used on spaceships to withstand high temperature changes? Is it something that can be crumpled into a small ball and when released expand back to it’s original size, like a memory foam? I think this is really interesting, but do not quite understand the potential uses for this new material and possible energy storage source.

It seems like every few weeks there is a “lab breakthrough” that is supposed to improve solar panel technology by make it cheaper and more efficient. Unfortunately all of these mini breakthroughs have seemingly amounted to…squat!?!?

Remember this one? http://news.rpi.edu/luwakkey/2507 Renssalear supposedly developed a coating for solar panels that would “…boost the amount of sunlight captured by solar panels and allow those panels to absorb the entire spectrum of sunlight from any angle, regardless of the sun’s position in the sky”. AWESOME…but that was 5 years ago. What is the status of this “game changing” technology? Why is it not already implemented and/or available commercially?

But things do improve. Several years ago I investigated solar for the house. I was told the efficiency was 11%. A month ago I again asked due to price reduction. Efficiency was 23%. The man said in another 3 years it will be 35-40%. It’s not exciting, doesn’t make headlines or “cool” like a tablet, but solar is continually improving.

This illustrates a common engineering problem. Years ago I decided to work on improving my VW mileage. So, I mailordered a number of after market devices that had been proven to result in 10-25% savings. The only problem was that total efficiency added up to 112%, which meant that every night I had to siphon gas out of the tank. What a nuisance! (inspired by “Ask Dr. Science”)

$1/watt = $1000/KW. At $0.10/KWhr this is 10,000 hrs of operation. Assuming an effective 4 hrs/day of operation this gives 2500 days or ~7 years for ROI. Shorter if you pay more for electricity. This is equivalent to an interest rate of about 10% (or greater if you pay more for electricity.)

‘Much higher, more efficient performance.’ Exciting! A percent here, a percent there, and eventually it adds up to real power. I hope someone is developing a sustainable way of recycling these exotic materials for their elements, so we do not have to pollute and heat the earth to continually extract them.